1. Field of the Invention
The present invention relates to information transmission means in an intermediate lens tube to be mounted between an interchangeable lens and a camera body. Said information transmission means serves to transmit an information signal from the interchangeable lens to the camera body when they are joined together.
2. Description of the Prior Art
When an intermediate lens tube is mounted between an interchangeable lens and a camera body, there occurs sometimes a vignetting phenomenon. By this phenomenon the bundle of light rays coming from the exit pupil of the interchangeable lens is vignetted. Such vignetting is caused by the intermediate lens tube case by case. In case that the intermediate lens tube is mounted between an interchangeable lens having a relatively large minimum F-number (which generally means a small lens aperture) and a camera body, it is seldom that vignetting is caused by the lens tube. However, in the case of interchangeable lenses having a relatively small minimum F-number (large in lens aperture), the lens tube is apt to cause vignetting. In this manner, for one and the same intermediate lens tube, it is a case by case question as to whether vignetting takes place or not, depending on the minimum F-number of the interchangeable lens with which the intermediated lens tube is connected. Considering many combinations of the same intermediate lens tube and various interchangeable lenses, it is possible preliminarily to determine a critical minimum F-number in respect of the occurrence of vignetting.
If an interchangeable lens having a smaller minimum F-number than such critical minimum F-number is connected to the intermediate lens tube, then a part of the exit light from the interchangeable lens, with its aperture being at or near its minimum F-number, will be blocked by the lens tube and therefore the quantity of photographing light reaching the focal plane in the camera will be substantially reduced as compared with the quantity of light passed through the diaphragm of the interchangeable lens.
On the other hand there is known in the art such interchangeable lens which has means for generating a signal informing of its minimum F-number. Such signal generating means is provided on the mount of the interchangeable lens. This F-number information signal is introduced into the exposure control part of the camera, for example, into an exposure meter, shutter speed priority automatic exposure control apparatus or programmed automatic exposure control apparatus. The input signal is processed in the exposure control part of the camera. Therefore, as readily understood, when the above mentioned interchangeable lens and intermediate lens tube are mounted in front of a camera provided with the above exposure control apparatus, the reduction of quantity of light caused by vignetting should be taken into consideration. Otherwise, maloperation of exposure control may be caused thereby.
For example, we can mention the case where the diaphragm of the interchangeable lens is controlled from the side of camera body when a picture is taken employing a combination of the above intermediate lens tube and interchangeable lens. In this case, although the control itself may be performed correctly, the information of diaphragm value then indicated in the finder of the camera can not be correct but show the operator incorrect information.
The same may be said of programmed control. In the case of programmed control, the combination of shutter speed and aperture value is selected and controlled in accordance with a predetermined program considering the change in luminance of an object. If above mentioned vignetting takes place, then the control will be carried out in accordance with a program shifted from the predetermined control program to the extent corresponding to the magnitude of vignetting. This shift generally occurs in the direction toward the side of low shutter speed. This is disadvantageous in particular when a known rear conversion lens is used as the intermediate lens tube. The rear conversion lens includes an optical system for converting focal length. Since the focal length is lengthened by the rear conversion lens, it becomes difficult to hold the camera stably at the time of picture taking. A stable holding of the camera may be possible so long as the shutter speed set at that time is relatively high. However, at a low shutter speed, the holding of camera is apt to be unstable. In this sense, the shift of exposure control program toward the side of low shutter speed is undesirable.